The mandatory use of brake lights on a vehicle allows the operator of a following car to govern his speed accordingly. Brake lights are far more likely to be perceived as indicating a change in speed of the vehicle than does the decrease in vehicle speed.
There are, of course, brake lights used which light when the brakes of a vehicle are applied. These lights use the battery power of the vehicle and are generally initiated through a simple relay and switch arrangement that creates power flow to the brake light when the brake pedal is depressed. Such brake lights do not illuminate when the brake pedal is not depressed, despite the fact that the vehicular speed may be decreasing.
Certain lights are illuminated, in some instances, when the brake pedal is not depressed but the foot of the operator is removed from the accelerator. For example, a red-amber-green combination light which is normally positioned in the back window of the vehicle operates to illuminate the green when the operator's foot is on the accelerator, the amber when the operator's foot is off the accelerator and the red when the brake pedal is depressed. It is again illuminated with switch and relay means.
Certain disadvantages are inherent in such systems. For example, in tractor-trailer rigs, long cables from the brake lights to the batteries and the relays and switches are vulnerable to damage. On a bicycle, where the change in bicycle speed is even less perceptible than a car, brake lights are not normally used and, of course, they will not illuminate if the brakes are not applied. Since a bicycle may be stopped with both hand and foot brakes, it would be necessary to install a switch for each with the concomitant necessity for extra wiring, etc., which is unsightly and complicated on a bicycle. On wheelchairs, similar considerations apply but it is desirable to have an indication when the speed is decreasing and not, necessarily, just when the brakes are applied.